Group I metabotropic glutamate receptors inhibit GABA release at interneuron-Purkinje cell synapses through endocannabinoid production

Low doses of cannabinoids enhance the antinociceptive effects of intracisternally administered mGluRs groups II and III agonists in formalin-induced TMJ nociception in rats

This study provides the first demonstration that central cannabinoids modulate the antinociceptive actions of metabotropic glutamate receptors (mGluRs) on formalin-induced temporomandibular joint (TMJ) nociception. Noxious scratching behavior induced by formalin injection in the TMJ was used as a model of pain. Intracisternal injection of 30mug of WIN 55,212-2, a non-subtype selective cannabinoid receptor agonist, attenuated the number of scratches by 75% as compared with the vehicle-treated group, whereas vehicle alone or 3 or 10 microg of WIN 55,212-2 had no effect. To explore the postulated interaction between central cannabinoid receptors and mGluRs, effects of combined administration of sub-analgesic doses of WIN 55,212-2 and group II or III mGluR agonists were tested. Group II or III mGluRs agonists were administered intracisternally 10 min after intracisternal administration of WIN 55,212-2. Neither 100 nmol APDC, a group II mGluRs agonist, nor L-AP4, a group III mGluR agonist, altered nociceptive behavior when given alone but significantly inhibited the formalin-induced nociceptive behavior in the presence of a sub-threshold dose ( 3microg) of WIN 55,212-2. The ED50 value of APDC or L-AP4 was significantly reduced upon co-treatment with WIN 55,212-2 than in the vehicle-treated group, highlighting the important therapeutic potential of the combined administration of group II or III mGluR agonists with cannabinoids to effectively treat inflammatory pain associated with the TMJ. Potentiating effects of group II or III mGluRs agonists will likely permit the administration of cannabinoids at doses that do not achieve significant accumulation to produce undesirable motor dysfunction.

Metaplastic control of the endocannabinoid system at inhibitory synapses in hippocampus

The modifiability of neuronal response plasticity is called "metaplasticity." In suppressing synaptic inhibition and facilitating induction of long-term excitatory synaptic plasticity, endocannabinoids (eCBs) act as agents of metaplasticity. We now report the discovery of a calcium-dependent mechanism that regulates eCB mobilization by metabotropic glutamate receptor (mGluR) activation. The switch-like mechanism primes cells to release eCBs and requires a transient rise in intracellular Ca2+ concentration ([Ca2+]i) but not concurrent activation of mGluRs. Conversely, short-term, [Ca2+]i-dependent eCB release can be persistently enhanced by mGluR activation. Hence, eCBs are also objects of metaplasticity, subject to higher levels of physiological control.

Brain regional differences in the effect of ethanol on GABA release from presynaptic terminals

Whereas ethanol has behavioral actions consistent with increased GABAergic function, attempts to demonstrate a direct enhancement of GABA-gated currents by ethanol have produced mixed results. Recent work has suggested that a part of the GABAergic profile of ethanol may result from enhanced GABA release from presynaptic terminals. The present study examines the effect of ethanol on GABA release in several brain regions to assess the regional nature of ethanol-induced GABA release. Whole-cell voltage clamp recording of spontaneous inhibitory postsynaptic currents (sIPSCs) from mechanically dissociated neurons and miniature inhibitory postsynaptic currents (mIPSCs) and paired-pulse ratio (PPR) from a slice preparation were used to quantify GABA release. Ethanol produced a concentration-dependent increase in the frequency of sIPSCs recorded from mechanically dissociated cerebellar Purkinje neurons and mIPSCs from substantia nigra neurons without having an effect on sIPSCs recorded from lateral septal or cerebrocortical neurons. This regional difference in the effect of ethanol on GABA release was confirmed with PPR recording from brain slices. These data indicate that ethanol can act on presynaptic terminals to increase GABA release in some brain regions while having little or no effect on GABA release in others. This regional difference is consistent with earlier in vivo studies in which ethanol affected neural activity and sensitivity to GABA in some, but not all, brain sites.

Potentiation of electrical and chemical synaptic transmission mediated by endocannabinoids

Endocannabinoids are well established as inhibitors of chemical synaptic transmission via presynaptic activation of the cannabinoid type 1 receptor (CB1R). Contrasting this notion, we show that dendritic release of endocannabinoids mediates potentiation of synaptic transmission at mixed (electrical and chemical) synaptic contacts on the goldfish Mauthner cell. Remarkably, the observed enhancement was not restricted to the glutamatergic component of the synaptic response but also included a parallel increase in electrical transmission. This effect involved the activation of CB1 receptors and was indirectly mediated via the release of dopamine from nearby varicosities, which in turn led to potentiation of the synaptic response via a cAMP-dependent protein kinase-mediated postsynaptic mechanism. Thus, endocannabinoid release can potentiate synaptic transmission, and its functional roles include the regulation of gap junction-mediated electrical synapses. Similar interactions between endocannabinoid and dopaminergic systems may be widespread and potentially relevant for the motor and rewarding effects of cannabis derivatives.

The phytocannabinoid Delta(9)-tetrahydrocannabivarin modulates inhibitory neurotransmission in the cerebellum

BACKGROUND AND PURPOSE

The phytocannabinoid Delta(9)-tetrahydrocannabivarin (Delta(9)-THCV) has been reported to exhibit a diverse pharmacology; here, we investigate functional effects of Delta(9)-THCV, extracted from Cannabis sativa, using electrophysiological techniques to define its mechanism of action in the CNS.

EXPERIMENTAL APPROACH

Effects of Delta(9)-THCV and synthetic cannabinoid agents on inhibitory neurotransmission at interneurone-Purkinje cell (IN-PC) synapses were correlated with effects on spontaneous PC output using single-cell and multi-electrode array (MEA) electrophysiological recordings respectively, in mouse cerebellar brain slices in vitro.

KEY RESULTS

The cannabinoid receptor agonist WIN 55,212-2 (WIN55) decreased miniature inhibitory postsynaptic current (mIPSC) frequency at IN-PC synapses. WIN55-induced inhibition was reversed by Delta(9)-THCV, and also by the CB(1) receptor antagonist AM251; Delta(9)-THCV or AM251 acted to increase mIPSC frequency beyond basal values. When applied alone, Delta(9)-THCV, AM251 or rimonabant increased mIPSC frequency. Pre-incubation with Delta(9)-THCV blocked WIN55-induced inhibition. In MEA recordings, WIN55 increased PC spike firing rate; Delta(9)-THCV and AM251 acted in the opposite direction to decrease spike firing. The effects of Delta(9)-THCV and WIN55 were attenuated by the GABA(A) receptor antagonist bicuculline methiodide.

CONCLUSIONS AND IMPLICATIONS

We show for the first time that Delta(9)-THCV acts as a functional CB(1) receptor antagonist in the CNS to modulate inhibitory neurotransmission at IN-PC synapses and spontaneous PC output. Delta(9)-THCV- and AM251-induced increases in mIPSC frequency beyond basal levels were consistent with basal CB(1) receptor activity. WIN55-induced increases in PC spike firing rate were consistent with synaptic disinhibition; whilst Delta(9)-THCV- and AM251-induced decreases in spike firing suggest a mechanism of PC inhibition.

Glutamate spillover modulates GABAergic synaptic transmission in the rat midbrain periaqueductal grey via metabotropic glutamate receptors and endocannabinoid signaling

Glutamate spillover regulates GABAergic synaptic transmission at several CNS synapses via presynaptic ionotropic and metabotropic glutamate receptors (mGluRs). We have previously demonstrated that activation of group I-III mGluRs inhibits GABAergic transmission in the midbrain periaqueductal gray (PAG), a region involved in organizing behavioral responses to threat, stress, and pain. Here, we examined the role of glutamate spillover in the modulation of GABAergic transmission in the PAG. Using whole-cell recordings from rat PAG slices, we found that evoked IPSCs were reduced by the nonspecific glutamate transport blockers DL-threo-beta-benzyloxyaspartic acid (TBOA) and L-trans-pyrrolidine-2,4-dicarboxylic acid, but not by the glial GLT1-specific blocker dihydrokainate. In contrast, TBOA had no effect on evoked IPSCs when glutamate uptake into the postsynaptic neuron was selectively impaired. TBOA increased the paired-pulse ratio of evoked IPSCs and reduced the rate but not the amplitude of spontaneous miniature IPSCs. The effect of TBOA on evoked IPSCs was abolished by the broad-spectrum mGluR antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (100 microM), reduced by the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and mimicked by the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the effects of both TBOA and DHPG were reduced by the cannabinoid CB1 receptor antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM251). Finally, although MPEP and AM251 had no effect on single evoked IPSCs, they increased evoked IPSCs during repetitive stimulation. These results indicate that neuronal glutamate transporters limit mGluR5 activation and endocannabinoid signaling, but may be overwhelmed during conditions of elevated glutamate release. Thus, neuronal glutamate transporters play a key role in regulating endocannabinoid-mediated cross talk between glutamatergic and GABAergic synapses within the PAG.

mGluR1-mediated facilitation of long-term potentiation at inhibitory synapses on a cerebellar Purkinje neuron

Synaptic plasticity has been studied extensively at excitatory synapses, whereas studies on plasticity at GABAergic inhibitory synapses have been limited. In the rat cerebellar cortex, postsynaptic depolarization of a Purkinje neuron (PN) induces long-term potentiation of GABA(A) receptor (GABA(A)R) responsiveness (termed rebound potentiation; RP). Induction of RP requires an increase in intracellular Ca(2+) concentration and resultant activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). We previously reported that GABA(B) receptor (GABA(B)R) activation coupled with depolarization suppresses RP induction by facilitating protein phosphatase 1 (PP-1)-mediated inhibition of CaMKII through down-regulation of cAMP-dependent protein kinase A (PKA) activity. Here, we examined the involvement of metabotropic glutamate receptor type 1 (mGluR1) in RP regulation. RP was monitored with the amplitudes of either the current responses to GABA or miniature inhibitory postsynaptic currents recorded from a PN in a primary culture or in a cerebellar slice. Inhibition of mGluR1 by an antagonist, 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate-ethyl-ester (CPCCOEt), prevented RP induction, which was abolished either by activation of adenylyl cyclase or by inhibition of PP-1. Furthermore, mGluR1 inhibition impaired depolarization-induced CaMKII activation. By contrast, activation of mGluR1 by the agonist (R,S)3,5-dihydroxyphenylglycine (DHPG) rescued RP induction from its suppression by GABA(B)R activation. The rescue was impaired either by inhibition of PKA or by facilitation of PP-1 activity. In addition, mGluR1 activation counteracted the GABA(B)R-mediated CaMKII inhibition. Taken together, these results suggest that mGluR1 activity counteracts GABA(B)R activity and contributes to RP induction through PKA activation, down-regulation of PP-1 and up-regulation of CaMKII.

Presynaptic modulation by endocannabinoids

Modulation of neurotransmitter release by G-protein-coupled receptors (GPCRs) is a prominent presynaptic mechanism for regulation of synaptic transmission. Activation of GPCRs located at the presynaptic terminal can decrease the probability of neurotransmitter release. This presynaptic depression involves activation of Gi/o-type G-proteins that mediate different inhibitory mechanisms, including inhibition of voltage-gated calcium channels, activation of potassium channels, and direct inhibition of the vesicle fusion process. A variety of neurotransmitters and modulatory agents can activate GPCRs that produce presynaptic depression. Among these are lipid metabolites that serve as agonists for GPCRs. The discovery of endocannabinoids and their cognate receptors, including the CB1 receptor, has stimulated intense investigation into the neurophysiological roles of these lipid metabolites. It is now clear that presynaptic depression is the major physiological role for the CB1 receptor. Endocannabinoids activate this receptor mainly via a retrograde signaling process in which these compounds are synthesized in and released from postsynaptic neuronal elements, and travel back to the presynaptic terminal to act on the CB1 receptor. This retrograde endocannabinoid modulation has been implicated in short-term synaptic depression, including suppression of excitatory or inhibitory transmission induced by postsynaptic depolarization and transient synaptic depression induced by activation of postsynaptic GPCRs during agonist treatment or synaptic activation. Endocannabinoids and the CB1 receptor also play a key role in one form of long-term synaptic depression (LTD) that involves a longlasting decrease in neurotransmitter release.

Developmental changes in agonist-induced retrograde signaling at parallel fiber-Purkinje cell synapses: role of calcium-induced calcium release

In cerebellar Purkinje cells (PCs), activation of postsynaptic mGluR1 receptors inhibits parallel fiber (PF) to PC synaptic transmission by retrograde signaling. However, results were conflicting with respect to whether endocannabinoids or glutamate (Glu) is the retrograde messenger involved. Experiments in cerebellar slices from 10- to 12-day-old rats and mice confirmed that suppression of PF-excitatory postsynaptic currents (EPSCs) by mGluR1 agonists was entirely blocked by cannabinoid receptor antagonists at this early developmental stage. In contrast, suppression of PF-EPSCs by mGluR1 agonists was only partly blocked by cannabinoid receptor antagonists in 18- to 22-day-old rats, and the remaining suppression was accompanied by an increase in paired-pulse facilitation. This endocannnabinoidindependent suppression of PF-EPSCs was potentiated by the Glu uptake inhibitor D-threo-beta-benzyloxyaspartate (D-TBOA) and blocked by the desensitizing kainate (KA) receptors agonist SYM 2081, by nonsaturating concentrations of 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX) [but not by GYKI 52466 hydrochloride (GYKI)] and by dialyzing PCs with guanosine 5'-[beta-thio]diphosphate (GDP-betaS). An endocannnabinoid-independent suppression of PF-EPSCs was also present in nearly mature wild-type mice but was absent in GluR6(-/-) mice. The endocannnabinoid-independent suppression of PF-EPSCs induced by mGluR1 agonists and the KA-dependent component of depolarization-induced suppression of excitation (DSE) were blocked by ryanodine acting at a presynaptic level. We conclude that retrograde release of Glu by PCs participates in mGluR1 agonist-induced suppression of PF-EPSCs at nearly mature PF-PC synapses and that Glu operates through activation of presynaptic KA receptors located on PFs and prolonged release of calcium from presynaptic internal calcium stores.

Input-specific plasticity at excitatory synapses mediated by endocannabinoids in the dentate gyrus

Endocannabinoids (eCBs) mediate transient and long-lasting synaptic plasticity in several brain structures. In the dentate gyrus, activation of the type 1 cannabinoid receptor (CB1R) by exogenous ligands reportedly depresses excitatory synaptic transmission. However, direct evidence of eCB signaling at excitatory synapses in this region has been lacking. Here, we demonstrate that eCB release can be induced by a brief postsynaptic depolarization of dentate granule cells (DGCs), which potently and transiently suppresses glutamatergic inputs from mossy cell interneurons (MCs) but not from entorhinal cortex via the lateral and medial perforant paths. This input-specific depolarization-induced suppression of excitation (DSE) is calcium-dependent and can be modulated by agonists of cholinergic and group I metabotropic glutamate receptors. Inhibiting the synthesis of 2-arachidonoyl glycerol (2-AG), one of the most abundant eCBs in the brain, by diacyglycerol lipase (DGL) does not abolish DSE. Moreover, preventing the breakdown of anandamide, the other main eCB, does not potentiate DSE. Thus, eCB signaling underlying DSE in the dentate does not require DGL activity and is unlikely to be mediated by anandamide. Finally, we find that manipulations known to induce eCB-LTD at other central synapses do not trigger LTD at MCF-DGC synapses.

Roles of phospholipase Cbeta and NMDA receptor in activity-dependent endocannabinoid release

Endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid receptors and cause various forms of short-term and long-term synaptic plasticity throughout the brain. Using hippocampal and cerebellar neurons, we have revealed that endocannabinoid release can be induced through two different pathways. One is independent of phospholipase Cbeta (PLCbeta) and driven by Ca(2+) elevation alone (Ca(2+)-driven endocannabinoid release, CaER), and the other is PLCbeta-dependent and driven by activation of G(q/11)-coupled receptors (receptor-driven endocannabinoid release, RER). CaER is induced by activation of either voltage-gated Ca(2+) channels or NMDA receptors. RER is functional even at resting Ca(2+) levels (basal RER), but markedly enhanced by a small Ca(2+) elevation (Ca(2+)-assisted RER). In Ca(2+)-assisted RER, PLCbeta serves as a coincidence detector of receptor activation and Ca(2+) elevation. We have also demonstrated that Ca(2+)-assisted RER is essential for the endocannabinoid release triggered by synaptic activity. Our anatomical data show that a set of receptors and enzymes required for RER are well organized so that the excitatory input can trigger RER effectively. Certain forms of spike-timing-dependent plasticity (STDP) are reported to depend on endocannabinoid signalling. The NMDA receptor and PLCbeta might play key roles in the endocannabinoid-dependent forms of STDP as coincidence detectors with different timing dependences.

Retrograde endocannabinoid inhibition of goldfish retinal cones is mediated by 2-arachidonoyl glycerol

A functional role for retinal endocannabinoids has not been determined. We characterized retrograde suppression of membrane currents of goldfish cones in a retinal slice. Whole-cell recordings were obtained from cone inner segments under voltage clamp. I(K(V)) was elicited by a depolarizing pulse to +54 mV from a holding potential of -70 mV. A fifty-millisecond puff of saline with 70 mM KCl or Group I mGluR agonist DHPG was applied through a pipette directly at a mixed rod/cone (Mb) bipolar cell body. The amplitude of I(K(V)) decreased 25% compared to the pre-puff control. Retrograde suppression of I(K(V)) was blocked by CB1 receptor antagonist, SR141716A. The FAAH inhibitor URB597 had no effect on the suppression of I(K(V)), whereas nimesulide, a COX-2 inhibitor, prolonged the effects of the K+ puff 10-fold. Orlistat, a blocker of 2-AG synthesis, blocked the effect of the K+ puff. Group I mGluR activation of Gq/11 was demonstrated in that a puff with DHPG decreased I(K(V)) of cones by 32%, an effect blocked by SR141716A. The effect of DHPG was not blocked by the mGluR5 antagonist MPEP, indicating involvement of mGluR1. The suppressive effect of the K+ puff vanished in a Ca2+-free, 2 mM Co2+ saline. TMB-8 or ryanodine, blocked the effect of DHPG, but not that of the K+ puff, showing that calcium influx or release from intracellular stores could mediate retrograde release. We suggest that retrograde suppression of cone I(K(V)) is mediated by Ca2+-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms: (1) voltage-dependent, mimicked by the K+ puff, that may be activated by the depolarizing ON response to light; (2) voltage-independent, occurring under ambient illumination, mediated by tonic mGluR1 activation. The negative feedback of this latter mechanism could regulate tonic glutamate release from cones within narrow limits, regardless of ambient illumination.

Endocannabinoids limit metabotropic glutamate 5 receptor-mediated synaptic inhibition of striatal principal neurons

Synaptic transmission in the striatum is regulated by metabotropic glutamate (mGlu) receptors through pre- and postsynaptic mechanisms. We investigated the involvement of mGlu 1 and 5 receptors in the control of both excitatory and inhibitory transmission in the striatum. The mGlu 1 and 5 receptor agonist 3,5-DHPG failed to affect glutamate transmission, while it caused a biphasic effect on GABA transmission, characterized by early increase and late decrease in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from striatal principal neurons. Both mGlu 1 and 5 receptors were involved in the early response to 3,5-DHPG, through membrane depolarization of striatal GABAergic interneurons and action potential generation. The 3,5-DHPG-mediated late depression of inhibitory inputs to striatal principal neurons was conversely secondary to mGlu 5 receptor activation and subsequent endocannabinoid release. In conclusion, we have identified an mGlu-dependent mechanism of GABA transmission regulation of potential relevance for physiological neuronal activity.

Ca2+-assisted receptor-driven endocannabinoid release: mechanisms that associate presynaptic and postsynaptic activities

Endogenous cannabinoids (endocannabinoids) serve as retrograde messengers at synapses in various regions of the brain. They are released from postsynaptic neurons and cause transient and long-lasting reduction of neurotransmitter release through activation of presynaptic cannabinoid receptors. Endocannabinoid release is induced either by increased postsynaptic Ca(2+) levels or by activation of G(q/11)-coupled receptors. When these two stimuli coincide, endocannabinoid release is markedly enhanced, which is attributed to the Ca(2+) dependency of phospholipase Cbeta (PLCbeta). This Ca(2+)-assisted receptor-driven endocannabinoid release is suggested to participate in various forms of synaptic plasticity, including short-term associative plasticity in the cerebellum and spike-timing-dependent long-term depression in the somatosensory cortex. In these forms of plasticity, PLCbeta seems to function as a coincident detector of presynaptic and postsynaptic activities.

Interactions of cannabidiol with endocannabinoid signalling in hippocampal tissue

The phytocannabinoid cannabidiol (CBD) possesses no psychotropic activity amid potentially beneficial therapeutic applications. We here characterized interactions between CBD (1 microM) and the endocannabinoid system in cultured rat hippocampal cells. The CBD-induced Ca2+ rise observed in neurons and glia was markedly reduced in the presence of the endogenous cannabinoid anandamide in neurons, with no alteration seen in glia. Neuronal CBD responses were even more reduced in the presence of the more abundant endocannabinoid 2-arachidonyl glycerol, this action was maintained in the presence of the CB1 receptor antagonist AM281 (100 nM). Neuronal CBD responses were also reduced by pre-exposure to glutamate, expected to increase endocannabinoid levels by increasing in [Ca2+]i. Application of AM281 at 1 microM elevated CBD-induced Ca2+ responses in both cell types, further confirming our finding that endocannabinoid-mediated signalling is negatively coupled to the action of CBD. However, upregulation of endogenous levels of endocannabinoids via inhibition of endocannabinoid hydrolysis (with URB597 and MAFP) could not be achieved under resting conditions. Because delta9-tetrahydrocannabinol did not mimic the endocannabinoid actions, and pertussis toxin treatment had no effect on CBD responses, we propose that the effects of AM281 were mediated via a constitutively active signalling pathway independent of CB1 signalling. Instead, signalling via G(q/11) and phospholipase C appears to be negatively coupled to CBD-induced Ca2+ responses, as the inhibitor U73122 enhanced CBD responses. Our data highlight the interaction between exogenous and endogenous cannabinoid signalling, and provide evidence for the presence of an additional pharmacological target, sensitive to endocannabinoids and to AM281.

Depolarization-induced Rapid Generation of 2-Arachidonoylglycerol, an Endogenous Cannabinoid Receptor Ligand, in Rat Brain Synaptosomes

2-arachidonoylglycerol (2-AG) is an endogenous ligand for the cannabinoid receptors with a variety of potent biological activities. In this study, we first examined the effects of potassium-induced depolarization on the level of 2-AG in rat brain synaptosomes. We found that a significant amount of 2-AG was generated in the synaptosomes following depolarization. Notably, depolarization did not affect the levels of other molecular species of monoacylglycerols. Furthermore, the level of anandamide was very low and did not change markedly following depolarization. It thus appeared that the depolarization-induced accelerated generation is a unique feature of 2-AG. We obtained evidence that phospholipase C is involved in the generation of 2-AG in depolarized synaptosomes: U73122, a phospholipase C inhibitor, markedly reduced the depolarization-induced generation of 2-AG, and the level of diacylglycerol was rapidly elevated following depolarization. A significant amount of 2-AG was released from synaptosomes upon depolarization. Interestingly, treatment of the synaptosomes with SR141716A, a CB1 receptor antagonist, augmented the release of glutamate from depolarized synaptosomes. These results strongly suggest that the endogenous ligand for the cannabinoid receptors, i.e. 2-AG, generated through increased phospholipid metabolism upon depolarization, plays an important role in attenuating glutamate release from the synaptic terminals by acting on the CB1 receptor.

Developmental changes in retrograde messengers involved in depolarization-induced suppression of excitation at parallel fiber-Purkinje cell synapses in rodents

At parallel fiber (PF) to Purkinje cell (PC) synapses, depolarization-induced suppression of excitation (DSE) and suppression of PF-excitatory postsynaptic currents (EPSCs) by activation of postsynaptic mGluR1 glutamate (Glu) receptors involve retrograde release of endocannabinoids. However, Levenes et al. suggested instead that Glu was the retrograde messenger in this latter case. Because the study by Levenes et al. was performed in nearly mature rats, whereas most others were performed in juvenile animals, DSE was re-investigated in juvenile versus nearly mature rats and mice. Indeed, DSE was preferred here to agonist-induced suppression of PF-EPSCs, to avoid possible indirect effects in this latter case. In 10- to 12-day-old rats, DSE of PF-EPSCs was entirely mediated through retrograde release of endocannabinoids. In 18- to 22-day-old-rats, DSE was partly resistant to CB1 cannabinoid receptor antagonists. The remaining component was potentiated by the Glu uptake inhibitor d-threo-beta-benzyloxyaspartate (d-TBOA) and blocked by the desensitizing kainate (KA) receptor agonist (2S,4R)-4-methylglutamic acid (SYM 2081). This SYM-2081-sensitive component of DSE was accompanied by a paired-pulse facilitation increase that was also potentiated by d-TBOA and blocked by SYM 2081. In nearly mature wild-type and GluR6 -/- mice, results fully confirmed the presence of an endocannabinoid-independent component of DSE that involves retrograde release of Glu and activation of presynaptic KA receptors including GluR6 receptor subunits. Therefore retrograde release of Glu by PCs participates to DSE at PF-PC synapses in nearly mature rodents but not in juvenile ones, and Glu probably operates through activation of presynaptic KA receptors that include GluR6 receptor subunits.

Endogenous mGluR activity suppresses GABAergic transmission in avian cochlear nucleus magnocellularis neurons

GABAergic transmission in the avian cochlear nucleus magnocellularis (NM) of the chick is subject to modulation by gamma-aminobutyric acid type B (GABA(B)) autoreceptors. Here, I investigated modulation of GABAergic transmission in NM by metabotropic glutamate receptors (mGluRs) with whole cell recordings in brain slice preparations. I found that tACPD, a nonspecific mGluR agonist, exerted dose-dependent suppression on evoked inhibitory postsynaptic currents (eIPSCs) in NM neurons. At concentrations of 100 or 200 microM, tACPD increased the failure rate of GABAergic transmission. Agonists for group I (3,5-DHPG, 200 microM), group II (DCG-IV, 2 microM), and group III (L-AP4, 10 microM) mGluRs produced a significant reduction in the amplitude of eIPSCs and a significant increase in failure rate, indicating the involvement of multiple mGluRs in this modulation. The frequency, but not the amplitude, of miniature IPSCs (mIPSCs) was decreased significantly by 3,5-DHPG or DCG-IV. Neither frequency nor amplitude of mIPSCs was affected by L-AP4. mGluR antagonists LY341495 (20 microM) plus CPPG (10 microM) significantly increased the amplitude of eIPSCs, indicating that endogenous mGluR activity suppresses GABA release to NM neurons. Furthermore, blockage of mGluRs increased GABA-evoked discharges recorded under physiological Cl(-) concentrations, whereas tACPD (100 microM) eliminated them. The results indicate that mGluRs play important roles in achieving balanced excitation and inhibition in NM and preserving fidelity of temporal information encoded by NM neurons.

Cortical efferent control of subcortical sensory neurons by synaptic disinhibition

A long-standing hypothesis predicts that pyramidal neurons of the cerebral cortex control the influx of sensory information at the level of primary sensory representations areas. Yet little is known about the cellular mechanisms governing selective attention to behaviorally relevant objects in space. Neurons in the superficial layers of the superior colliculus are notably involved in this process, and they are directly targeted by retinal and cortical afferents. To study long-term and short-term effects of the visual cortex (VC) on subcortical visual neurons we established an in vitro model of the developing cortico-tectal projection. To this end, cortical explants expressing Green Fluorescent Protein were allowed to form connections with non-labeled dissociated tectal neurons. The presence of VC explants led to an enhancement of tectal activity by 2 mechanisms. First, glutamatergic input was increased. Second, intrinsic GABAergic inhibition was suppressed. The latter effect was shown to be acute and mediated through postsynaptic metabotropic glutamate receptor activation, G-protein acitivity, and endocannabinoid receptor activation. The VC-induced disinhibition was readily reversed by application of an mGluR antagonist. However, high-frequency activation of the glutamatergic cortico-tectal input turned the labile disinhibition into a persistent suppression of inhibition.

Forebrain-specific inactivation of Gq/G11 family G proteins results in age-dependent epilepsy and impaired endocannabinoid formation

Metabotropic receptors coupled to Gq/G11 family G proteins critically contribute to nervous system functions by modulating synaptic transmission, often facilitating excitation. We investigated the role of Gq/G11 family G proteins in the regulation of neuronal excitability in mice that selectively lack the alpha-subunits of Gq and G11, G alpha q and G alpha 11, respectively, in forebrain principal neurons. Surprisingly, mutant mice exhibited increased seizure susceptibility, and the activation of neuroprotective mechanisms was impaired. We found that endocannabinoid levels were reduced under both basal and excitotoxic conditions and that increased susceptibility to kainic acid could be normalized by the enhancement of endocannabinoid levels with an endocannabinoid reuptake inhibitor, while the competitive cannabinoid type 1 receptor antagonist SR141716A did not cause further aggravation. These findings indicate that Gq/G11 family G proteins negatively regulate neuronal excitability in vivo and suggest that impaired endocannabinoid formation in the absence of Gq/G11 contributes to this phenotype.

Depolarization-induced retrograde synaptic inhibition in the mouse cerebellar cortex is mediated by 2-arachidonoylglycerol

Endocannabinoids acting on CB(1) cannabinoid receptors are involved in short- and long-term depression of synaptic transmission. The aim of the present study was to determine which endocannabinoid, anandamide or 2-arachidonoylglycerol (2-AG), is involved in depolarization-induced suppression of inhibition (DSI) in the cerebellar cortex, which is the most widely studied form of short-term depression. Depolarization of Purkinje cells in the mouse cerebellum led to an increase in intracellular calcium concentration and to suppression of the inhibitory input to these neurons (i.e. DSI occurred). Orlistat and RHC80267, two blockers of sn-1-diacylglycerol lipase, the enzyme catalysing 2-AG formation, abolished DSI by acting downstream of calcium influx. In contrast, DSI occurred also in the presence of a phospholipase C inhibitor. Intact operation of the calcium-dependent messengers calmodulin and Ca(2+)-calmodulin-dependent protein kinase II were necessary for DSI. DSI was potentiated by an inhibitor of the main 2-AG-degrading enzyme, monoacylglycerol lipase. Interference with the anandamide metabolizing enzyme, fatty acid amide hydrolase, did not modify DSI. Thus, three kinds of observations identified 2-AG as the endocannabinoid involved in DSI in the mouse cerebellum: DSI was abolished by diacylglycerol lipase inhibitors; DSI was potentiated by a monoglyceride lipase inhibitor; and DSI was not changed by an inhibitor of fatty acid amide hydrolase. Further experiments indicated that 2-AG is the endocannabinoid mediating short-term retrograde signalling also at other synapses: orlistat abolished DSI in the rat cerebellum, DSI in the mouse substantia nigra pars reticulata and depolarization-induced suppression of excitation in the mouse cerebellum.

Retrograde endocannabinoid signaling in the cerebellar cortex

The regulation of Purkinje cell activity is important for motor behavior and motor learning. As the sole output cell of the cerebellar cortex, Purkinje cell firing is controlled by parallel fibers and climbing fiber synapses, and by inhibitory interneurons. Depolarization of Purkinje cells evokes endocannabinoid release that activates cannabinoid CB1 receptors expressed on boutons of its synaptic inputs to transiently decrease neurotransmitter release. In addition, associative activation of the excitatory inputs can liberate endocannabinoids to decrease synaptic strength for a prolonged duration. Here we review the different mechanisms of evoking endocannabinoid release and discuss the physiological role of endocannabinoids in mediating global modulation of synaptic strength, localized short-term associative plasticity and cerebellar long term depression.

Endocannabinoid-mediated synaptic plasticity in the CNS

Changes in synaptic efficacy are thought to be crucial to experience-dependent modifications of neural function. The diversity of mechanisms underlying these changes is far greater than previously expected. In the last five years, a new class of use-dependent synaptic plasticity that requires retrograde signaling by endocannabinoids (eCB) and presynaptic CB1 receptor activation has been identified in several brain structures. eCB-mediated plasticity encompasses many forms of transient and long-lasting synaptic depression and is found at both excitatory and inhibitory synapses. In addition, eCBs can modify the inducibility of non-eCB-mediated forms of plasticity. Thus, the eCB system is emerging as a major player in synaptic plasticity. Given the wide distribution of CB1 receptors in the CNS, the list of brain structures and synapses expressing eCB-mediated plasticity is likely to expand.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Localization of diacylglycerol lipase-alpha around postsynaptic spine suggests close proximity between production site of an endocannabinoid, 2-arachidonoyl-glycerol, and presynaptic cannabinoid CB1 receptor

2-arachidonoyl-glycerol (2-AG) is an endocannabinoid that is released from postsynaptic neurons, acts retrogradely on presynaptic cannabinoid receptor CB1, and induces short- and long-term suppression of transmitter release. To understand the mechanisms of the 2-AG-mediated retrograde modulation, we investigated subcellular localization of a major 2-AG biosynthetic enzyme, diacylglycerol lipase-alpha (DAGLalpha), by using immunofluorescence and immunoelectron microscopy in the mouse brain. In the cerebellum, DAGLalpha was predominantly expressed in Purkinje cells. DAGLalpha was detected on the dendritic surface and occasionally on the somatic surface, with a distal-to-proximal gradient from spiny branchlets toward somata. DAGLalpha was highly concentrated at the base of spine neck and also accumulated with much lower density on somatodendritic membrane around the spine neck. However, DAGLalpha was excluded from the main body of spine neck and head. In hippocampal pyramidal cells, DAGLalpha was also accumulated in spines. In contrast to the distribution in Purkinje cells, DAGLalpha was distributed in the spine head, neck, or both, whereas somatodendritic membrane was labeled very weakly. These results indicate that DAGLalpha is essentially targeted to postsynaptic spines in cerebellar and hippocampal neurons, but its fine distribution within and around spines is differently regulated between the two neurons. The preferential spine targeting should enable efficient 2-AG production on excitatory synaptic activity and its swift retrograde modulation onto nearby presynaptic terminals expressing CB1. Furthermore, different fine localization within and around spines suggests that the distance between postsynaptic 2-AG production site and presynaptic CB1 is differentially controlled depending on neuron types.

Neuroanatomical Distribution of Cannabinoid Receptor Gene Expression in the Brain of the Rough-Skinned Newt, Taricha granulosa

Type I cannabinoid receptor (CB1) is a G-protein coupled receptor with a widespread distribution in the central nervous system in mammals. In a urodele amphibian, the rough-skinned newt (Taricha granulosa), recent evidence indicates that endogenous cannabinoids (endocannabinoids) mediate behavioral responses to acute stress and electrophysiological responses to corticosterone. To identify possible sites of action for endocannabinoids, in situ hybridization using a gene and species specific cRNA probe was used to label CB1 mRNA in brains of male T. granulosa. Labeling of CB1 mRNA in the telencephalon was observed in the olfactory bulb and all areas of the pallium, as well as the bed nucleus of the stria terminalis and nucleus amygdalae dorsolateralis. The labeling of CB1 mRNA was also found in regions of the preoptic area, thalamus, midbrain tegmentum and tectum, cerebellum, and the stratum griseum of the hindbrain. A notable difference in CB1 labeling between this amphibian and mammals is the abundance of labeling in areas associated with olfaction (anterior olfactory nuclei, nucleus amygdalae dorsolateralis, and lateral pallium), which hints that endocannabinoids might modulate responses to odors as well as pheromones. This widespread distribution of CB1 labeling, particularly in sensory and motor control centers, fits with prior results showing that endocannabinoids modulate sensorimotor processing and behavioral output in this species. The distribution of CB1 in the brain of T. granulosa was in many of the same sites previously observed in the brain of the anuran amphibian, Xenopus laevis, as well as those of different species of mammals, suggesting that endocannabinoid signaling pathways are conserved.

Integration of endocannabinoid and leptin signaling in an appetite-related neural circuit

Recently developed therapeutics for obesity, targeted against cannabinoid receptors, result in decreased appetite and sustained weight loss. Prior studies have demonstrated CB1 receptors (CB1Rs) and leptin modulation of cannabinoid synthesis in hypothalamic neurons. Here, we show that depolarization of perifornical lateral hypothalamus (LH) neurons elicits a CB1R-mediated suppression of inhibition in local circuits thought to be involved in appetite and "natural reward." The depolarization-induced decrease in inhibitory tone to LH neurons is blocked by leptin. Leptin inhibits voltage-gated calcium channels in LH neurons via the activation of janus kinase 2 (JAK2) and of mitogen-activated protein kinase (MAPK). Leptin-deficient mice are characterized by both an increase in steady-state voltage-gated calcium currents in LH neurons and a CB1R-mediated depolarization-induced suppression of inhibition that is 6-fold longer than that in littermate controls. Our data provide direct electrophysiological support for the involvement of endocannabinoids and leptin as modulators of hypothalamic circuits underlying motivational aspects of feeding behavior.

Kinetic, pharmacological and activity-dependent separation of two Ca2+ signalling pathways mediated by type 1 metabotropic glutamate receptors in rat Purkinje neurones

Type 1 metabotropic glutamate receptors (mGluR1) in Purkinje neurones (PNs) are important for motor learning and coordination. Here, two divergent mGluR1 Ca2+-signalling pathways and the associated membrane conductances were distinguished kinetically and pharmacologically after activation by 1-ms photorelease of L-glutamate or by bursts of parallel fibre (PF) stimulation. A new, mGluR1-mediated transient K+ conductance was seen prior to the slow EPSC (sEPSC). It was seen only in PNs previously allowed to fire spontaneously or held at depolarized potentials for several seconds and was slowly inhibited by agatoxin IVA, which blocks P/Q-type Ca2+ channels. It peaked in 148 ms, had well-defined kinetics and, unlike the sEPSC, was abolished by the phospholipase C (PLC) inhibitor U73122. It was blocked by the BK Ca2+-activated K+ channel blocker iberiotoxin and unaffected by apamin, indicating selective activation of BK channels by PLC-dependent store-released Ca2+. The K+ conductance and underlying transient Ca2+ release showed a highly reproducible delay of 99.5 ms following PF burst stimulation, with a precision of 1-2 ms in repeated responses of the same PN, and a subsequent fast rise and fall of Ca2+ concentration. Analysis of Ca2+ signals showed that activation of the K+ conductance by Ca2+ release occurred in small dendrites and subresolution structures, most probably spines. The results show that PF burst stimulation activates two pathways of mGluR1 signalling in PNs. First, transient, PLC-dependent Ca2+ release from stores with precisely reproducible timing and second, slower Ca2+ influx in the cation-permeable sEPSC channel. The priming by prior Ca2+ influx in P/Q-type Ca2+ channels may determine the path of mGluR1 signalling. The precise timing of PLC-mediated store release may be important for interactions of PF mGluR1 signalling with other inputs to the PN.

Synaptically driven endocannabinoid release requires Ca2+-assisted metabotropic glutamate receptor subtype 1 to phospholipase Cbeta4 signaling cascade in the cerebellum

Endocannabinoids mediate retrograde signaling and modulate synaptic transmission in various regions of the CNS. Depolarization-induced elevation of intracellular Ca2+ concentration causes endocannabinoid-mediated suppression of excitatory/inhibitory synaptic transmission. Activation of G(q/11)-coupled receptors including group I metabotropic glutamate receptors (mGluRs) also causes endocannabinoid-mediated suppression of synaptic transmission. However, precise mechanisms of endocannabinoid production initiated by physiologically relevant synaptic activity remain to be determined. To address this problem, we made whole-cell recordings from Purkinje cells (PCs) in mouse cerebellar slices and examined their excitatory synapses arising from climbing fibers (CFs) and parallel fibers (PFs). We first characterized three distinct modes to induce endocannabinoid release by analyzing CF to PC synapses. The first mode is strong activation of mGluR subtype 1 (mGluR1)-phospholipase C (PLC) beta4 cascade without detectable Ca2+ elevation. The second mode is Ca2+ elevation to a micromolar range without activation of the mGluR1-PLCbeta4 cascade. The third mode is the Ca2+-assisted mGluR1-PLCbeta4 cascade that requires weak mGluR1 activation and Ca2+ elevation to a submicromolar range. By analyzing PF to PC synapses, we show that the third mode is essential for effective endocannabinoid release from PCs by excitatory synaptic activity. Furthermore, our biochemical analysis demonstrates that combined weak mGluR1 activation and mild depolarization in PCs effectively produces 2-arachidonoylglycerol (2-AG), a candidate of endocannabinoid, whereas either stimulus alone did not produce detectable 2-AG. Our results strongly suggest that under physiological conditions, excitatory synaptic inputs to PCs activate the Ca2+-assisted mGluR1-PLCbeta4 cascade, and thereby produce 2-AG, which retrogradely modulates synaptic transmission to PCs.

α-Adrenoceptive Dual Modulation of Inhibitory GABAergic Inputs to Purkinje Cells in the Mouse Cerebellum

Noradrenaline (NA) modulates synaptic transmission in various sites of the CNS. In the cerebellar cortex, several studies have revealed that NA enhances inhibitory synaptic transmission by β-adrenoceptor–and cyclic AMP–dependent pathways. However, the effects of α-adrenoceptor activation on cerebellar inhibitory neurotransmission have not yet been fully elucidated. Therefore we investigated the effects of the α1- or α2-adrenoceptor agonist on inhibitory postsynaptic currents (IPSCs) recorded from mouse Purkinje cells (PCs). We found that the nonselective α-adrenoceptor agonist 6-fluoro-norepinephrine increased both the frequency and amplitude of spontaneous IPSCs (sIPSCs). This enhancement was mostly mimicked by the selective α1-adrenoceptor agonist phenylephrine (PE). PE also enhanced the amplitude of evoked IPSCs (eIPSCs) and increased the frequency but not the amplitude of miniature IPSCs (mIPSCs). Moreover, PE decreased the paired-pulse ratio of eIPSCs and did not change γ-aminobutyric acid (GABA) receptor sensitivity in PCs. Conversely, the selective α2-adrenoceptor agonist clonidine significantly reduced both the frequency and the amplitude of sIPSCs. Neither eIPSCs nor mIPSCs were affected by clonidine. Furthermore, presynaptic cell-attached recordings showed that spontaneous activity of GABAergic interneurons was enhanced by PE but reduced by clonidine. These results suggest that NA enhances inhibitory neurotransmitter release by α1-adrenoceptors, which are expressed in presynaptic terminals and somatodendritic domains, whereas NA suppresses the excitability of interneurons by α2-adrenoceptors, which are expressed in presynaptic somatodendritic domains. Thus cerebellar α-adrenoceptors play roles in a presynaptic dual modulation of GABAergic inputs from interneurons to PCs, thereby providing a likely mechanism for the fine-tuning of information flow in the cerebellar cortex.

Dopamine modulation of state-dependent endocannabinoid release and long-term depression in the striatum

Endocannabinoids are important mediators of short- and long-term synaptic plasticity, but the mechanisms of endocannabinoid release have not been studied extensively outside the hippocampus and cerebellum. Here, we examined the mechanisms of endocannabinoid-mediated long-term depression (eCB-LTD) in the dorsal striatum, a brain region critical for motor control and reinforcement learning. Unlike other cell types, strong depolarization of medium spiny neurons was not sufficient to yield detectable endocannabinoid release. However, when paired with postsynaptic depolarization sufficient to activate L-type calcium channels, activation of postsynaptic metabotropic glutamate receptors (mGluRs), either by high-frequency tetanic stimulation or an agonist, induced eCB-LTD. Pairing bursts of afferent stimulation with brief subthreshold membrane depolarizations that mimicked down-state to up-state transitions also induced eCB-LTD, which not only required activation of mGluRs and L-type calcium channels but also was bidirectionally modulated by dopamine D2 receptors. Consistent with network models, these results demonstrate that dopamine regulates the induction of a Hebbian form of long-term synaptic plasticity in the striatum. However, this gating of plasticity by dopamine is accomplished via an unexpected mechanism involving the regulation of mGluR-dependent endocannabinoid release.

Behaviorally relevant endocannabinoid action in hippocampus: dependence on temporal summation of multiple inputs

Endocannabinoids have been shown to mediate depolarization-induced suppression of GABAergic inhibition (DSI), possibly via release and retrograde diffusion following moderate to severe depolarization of hippocampal pyramidal neurons. However, it is not clear how hippocampal neurons, which have relatively low firing rates in vivo, achieve the degree of depolarization required to release endocannabinoids. Here it is demonstrated that DSI is not dependent on the occurrence of action potentials in the postsynaptic neuron, but is mediated by depolarization-induced calcium entry via voltage-controlled calcium channels (VCCs). The optimal level of calcium entry, and subsequent DSI, are directly related to the frequency of depolarizing pulses, which differs between immature and adult hippocampus. However, it is shown via modeled spike train inputs that the frequency dependence of DSI is overcome if two or more convergent spike trains from different neurons with normal in vivo firing rates converge and overlap in time. In these modeled circumstances, endocannabinoid-mediated DSI occurs most often when converging synaptic inputs from multiple neurons fire in synchrony to allow temporal summation of local membrane events in postsynaptic cells to exceed threshold for calcium entry. It is therefore possible that such suppression of inhibition would only occur during the time that recipient hippocampal neurons receive multiple coincident excitatory synaptic inputs.

Cannabinoid receptor signaling

The cannabinoid receptor family currently includes two types: CB1, characterized in neuronal cells and brain, and CB2, characterized in immune cells and tissues. CB1 and CB2 receptors are members of the superfamily of seven-transmembrane-spanning (7-TM) receptors, having a protein structure defined by an array of seven membrane-spanning helices with intervening intracellular loops and a C-terminal domain that can associate with G proteins. Cannabinoid receptors are associated with G proteins of the Gi/o family (Gi1, 2 and 3, and Go1 and 2). Signal transduction via Gi inhibits adenylyl cyclase in most tissues and cells, although signaling via Gs stimulates adenylyl cyclase in some experimental models. Evidence exists for cannabinoid receptor-mediated Ca2+ fluxes and stimulation of phospholipases A and C. Stimulation of CB1 and CB2 cannabinoid receptors leads to phosphorylation and activation of p42/p44 mitogen-activated protein kinase (MAPK), p38 MAPK and Jun N-terminal kinase (JNK) as signaling pathways to regulate nuclear transcription factors. The CB1 receptor regulates K+ and Ca2+ ion channels, probably via Go. Ion channel regulation serves as an important component of neurotransmission modulation by endogenous cannabinoid compounds released in response to neuronal depolarization. Cannabinoid receptor signaling via G proteins results from interactions with the second, third and fourth intracellular loops of the receptor. Desensitization of signal transduction pathways that couple through the G proteins probably entails phosphorylation of critical amino acid residues on these intracellular surfaces.

Calcium and endocannabinoids in the modulation of inhibitory synaptic transmission

Synapses in the central nervous system can be highly plastic devices, being able to modify their efficacy in relaying information in response to several factors. Calcium ions are often fundamental in triggering synaptic plasticity. Here, we will shortly review the effects induced by postsynaptic increases of calcium concentration at GABAergic and glycinergic synapses. Both postsynaptic and presynaptic mechanisms mediating changes in synaptic strength will be examined. Particular attention will be devoted to phenomena of retrograde signaling and, specifically, to the recently discovered role, played by the endocannabinoid system in retrograde synaptic modulation.

Patterned expression of Purkinje cell glutamate transporters controls synaptic plasticity

Glutamate transporters are responsible for clearing synaptically released glutamate from the extracellular space. If expressed at high enough densities, transporters can prevent activation of extrasynaptic receptors by rapidly lowering glutamate concentrations to insignificant levels. We find that synaptic activation of metabotropic glutamate receptors expressed by Purkinje cells is prevented in regions of rat cerebellum where the density of the glutamate transporter EAAT4 is high. The consequences of metabotropic receptor stimulation, including activation of a depolarizing conductance, cannabinoid-mediated presynaptic inhibition and long-term depression, are also limited in Purkinje cells expressing high levels of EAAT4. We conclude that neuronal uptake sites must be overwhelmed by glutamate to activate perisynaptic metabotropic glutamate receptors. Regional differences in glutamate transporter expression affect the degree of metabotropic glutamate receptor activation and therefore regulate synaptic plasticity.

A role for cannabinoid CB1 receptors in mood and anxiety disorders

Mood and anxiety disorders, the most prevalent of the psychiatric disorders, cause immeasurable suffering worldwide. Despite impressive advances in pharmacological therapies, improvements in efficacy and side-effect profiles are needed. The present literature review examines the role that the endocannabinoid system may play in these disorders and the potential value of targeting this system in the search for novel and improved medications. Cannabis and its major psychoactive component (-)-trans-delta9-tetrahydrocannabinol, have profound effects on mood and can modulate anxiety and mood states. Cannabinoid receptors and other protein targets in the central nervous system (CNS) that modulate endocannabinoid function have been described. The discovery of selective modulators of some of these sites that increase or decrease endocannabinoid neurotransmission, primarily through the most prominent of the cannabinoid receptors in the CNS, the CB1 receptors, combined with transgenic mouse technology, has enabled detailed investigations into the role of these CNS sites in the regulation of mood and anxiety states. Although data point to the involvement of the endocannabinoid system in anxiety states, the pharmacological evidence seems contradictory: both anxiolytic- and anxiogenic-like effects have been reported with both endocannabinoid neurotransmission enhancers and blockers. Due to advances in the development of selective compounds directed at the CB1 receptors, significant progress has been made on this target. Recent biochemical and behavioural findings have demonstrated that blockade of CB1 receptors engenders antidepressant-like neurochemical changes (increases in extracellular levels of monoamines in cortical but not subcortical brain regions) and behavioural effects consistent with antidepressant/antistress activity in rodents.

The wake-promoting peptide orexin-B inhibits glutamatergic transmission to dorsal raphe nucleus serotonin neurons through retrograde endocannabinoid signaling

The wake-promoting neuropeptides orexins (hypocretins) play a crucial role in controlling neuronal excitability and synaptic transmission in the CNS. In this study, using whole-cell patch-clamp recordings in an acute dorsal raphe nucleus (DRN) slice preparation, we report that orexin B (Orx-B) depresses the evoked glutamate-mediated synaptic currents in DRN 5-HT neurons. The Orx-B-induced depression is accompanied by an increase in the paired-pulse ratio and the coefficient of variance, suggesting a presynaptic site of action. Orx-B also reduces the frequency but not the amplitude of miniature EPSCs, indicating that depression of glutamatergic transmission is mediated by a decrease in glutamate release. Surprisingly, the Orx-B-induced inhibition of glutamatergic transmission is abolished by postsynaptic inhibition of G-protein signaling with GDPbetaS, suggesting that this effect is signaled by postsynaptic orexin receptors and expressed presynaptically, presumably through a retrograde messenger. Interestingly, the Orx-B-induced depression of glutamate release is mimicked and occluded by the cannabinoid receptor agonist WIN 55,212-2, and is abolished by the CB1 cannabinoid receptor antagonist AM 251. These results imply that the Orx-B-induced depression of glutamatergic transmission to DRN 5-HT neurons is mediated by retrograde endocannabinoid release. Examination of downstream signaling pathways involved in this response indicates that the effect of Orx-B requires the activation of phospholipase C and DAG lipase enzymatic pathways but not a rise in postsynaptic intracellular calcium. Therefore, our findings reveal a previously unsuspected mechanism by which postsynaptic orexin receptors can modulate glutamatergic synaptic transmission to DRN 5-HT neurons.

Stimulation of endocannabinoid formation in brain slice cultures through activation of group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate (mGlu) receptors drives the endocannabinoid system to cause both short- and long-term changes of synaptic strength in the striatum, hippocampus, and other brain areas. Although there is strong electrophysiological evidence for a role of endocannabinoid release in mGlu receptor-dependent plasticity, the identity of the endocannabinoid transmitter mediating this phenomenon remains undefined. In this study, we show that activation of group I mGlu receptors triggers the biosynthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG), but not anandamide, in primary cultures of corticostriatal and hippocampal slices prepared from early postnatal rat brain. Pharmacological studies suggest that 2-AG biosynthesis is initiated by activation of mGlu5 receptors, is catalyzed by phospholipase C (PLC) and 1,2-diacylglycerol lipase (DGL) activities, and is dependent on intracellular Ca2+ ions. Realtime polymerase chain reaction and immunostaining analyses indicate that DGL-beta is the predominant DGL isoform expressed in corticostriatal and hippocampal slices and that this enzyme is highly expressed in striatal neurons, where it is colocalized with PLC-beta1. The results suggest that 2-AG is a primary endocannabinoid mediator of mGlu receptor-dependent neuronal plasticity.

Prefrontal cortex stimulation induces 2-arachidonoyl-glycerol-mediated suppression of excitation in dopamine neurons

Endocannabinoids form a novel class of retrograde messengers that modulate short- and long-term synaptic plasticity. Depolarization-induced suppression of excitation (DSE) and inhibition (DSI) are the best characterized transient forms of endocannabinoid-mediated synaptic modulation. Stimulation protocols consisting of long-lasting voltage steps to the postsynaptic cell are routinely used to evoke DSE-DSI. Little is known, however, about more physiological conditions under which these molecules are released in vitro. Moreover, the occurrence in vivo of such forms of endocannabinoid-mediated modulation is still controversial. Here we show that physiologically relevant patterns of synaptic activity induce a transient suppression of excitatory transmission onto dopamine neurons in vitro. Accordingly, in vivo endocannabinoids depress the increase in firing and bursting activity evoked in dopamine neurons by prefrontal cortex stimulation. This phenomenon is selectively mediated by the endocannabinoid 2-arachidonoyl-glycerol (2-AG), which activates presynaptic cannabinoid type 1 receptors. 2-AG synthesis involves activation of metabotropic glutamate receptors and Ca2+ mobilization from intracellular stores. These findings indicate that dopamine neurons release 2-AG to shape afferent activity and ultimately their own firing pattern. This novel endocannabinoid-mediated self-regulatory role of dopamine neurons may bear relevance in the pathogenesis of neuropsychiatric disorders such as schizophrenia and addiction.

Circuitry for associative plasticity in the amygdala involves endocannabinoid signaling

Endocannabinoids are crucial for the extinction of aversive memories, a process that considerably involves the amygdala. Here, we show that low-frequency stimulation of afferents in the lateral amygdala with 100 pulses at 1 Hz releases endocannabinoids postsynaptically from neurons of the basolateral amygdala of mice in vitro and thereby induces a long-term depression of inhibitory GABAergic synaptic transmission (LTDi) via a presynaptic mechanism. Lowering inhibitory synaptic transmission significantly increases the amplitude of excitatory synaptic currents in principal neurons of the central nucleus, which is the main output site of the amygdala. LTDi involves a selective mGluR1 (metabotropic glutamate receptor 1)-mediated calcium-independent mechanism and the activation of the adenylyl cyclase-protein kinase A pathway. LTDi is abolished by the cannabinoid type 1 (CB1) receptor antagonist SR141716A and cannot be evoked in CB1 receptor-deficient animals. LTDi is significantly enhanced in mice lacking the anandamide-degrading enzyme fatty acid amide hydrolase. The present findings show for the first time that mGluR activation induces a retrograde endocannabinoid signaling via activation of the adenylyl cyclase-protein kinase A pathway and the release of anandamide. Furthermore, the results indicate that anandamide decreases the activity of inhibitory interneurons in the amygdala. This disinhibition increases the activity of common output neurons and could provide a prerequisite for extinction by formation of new memory.

Associative short-term synaptic plasticity mediated by endocannabinoids

Associative learning is important on rapid timescales, but no suitable form of short-term plasticity has been identified that is both associative and synapse specific. Here, we assess whether endocannabinoids can mediate such plasticity. In the cerebellum, bursts of parallel fiber (PF) activity evoke endocannabinoid release from Purkinje cell dendrites that results in retrograde synaptic inhibition lasting seconds. We find that the powerful climbing fiber (CF) to Purkinje cell synapse regulates this inhibition. Compared to PF stimulation alone, coactivation of PF and CF synapses greatly enhanced endocannabinoid-mediated inhibition of PF synapses. Retrograde inhibition was restricted to PFs activated within several hundred milliseconds of CF activation. This associative plasticity reflects two aspects of calcium-dependent endocannabinoid release. First, PF-mediated activation of metabotropic glutamate receptors locally reduced the dendritic calcium levels required for endocannabinoid release. Second, CF and PF coactivation evoked localized supralinear dendritic calcium signals. Thus, endocannabinoids mediate transient associative synaptic plasticity.

NMDA receptors induce somatodendritic secretion in hypothalamic neurones of lactating female rats

Many neurones in the mammalian brain are known to release the content of their vesicles from somatodendritic locations. These vesicles usually contain retrograde messengers that modulate network properties. The back-propagating action potential is thought to be the principal physiological stimulus that evokes somatodendritic release. In contrast, here we show that calcium influx through NMDA receptor (NMDAR) channels, in the absence of postsynaptic cell firing, is also able to induce vesicle fusion from non-synaptic sites in nucleated outside-out patches of dorsomedial supraoptic nucleus (SON) neurones of adult female rats, in particular during their reproductive stages. The physiological significance of this mechanism was characterized in intact brain slices, where NMDAR-mediated release of oxytocin was shown to retrogradely inhibit presynaptic GABA release, in the absence of postsynaptic cell firing. This implies that glutamatergic synaptic input in itself is sufficient to elicit the release of oxytocin, which in turn acts as a retrograde messenger leading to the depression of nearby GABA synapses. In addition, we found that during lactation, when oxytocin demand is high, NMDA-induced oxytocin release is up-regulated compared to that in non-reproductive rats. Thus, in the hypothalamus, local signalling back and forth between pre- and postsynaptic compartments and between different synapses may occur independently of the firing activity of the postsynaptic neurone.

Immunohistochemical localization of cannabinoid CB1 receptor in inhibitory interneurons in the cerebellum

Inhibitory interneurons terminating on Purkinje cell dendrites contribute to cannabinoid-mediated cerebellar plasticity, consistent with the intense expression of cannabinoid CB1 receptor protein in the cerebellar molecular layer. CB1 labelling in the molecular layer has been attributed to parallel fibers originating from granule cells, climbing fibers originating in the inferior olive, and inhibitory interneurons in the deep molecular layer (basket cells). However, the cellular distribution of CB1 in the cerebellar molecular layer has remained poorly understood. We used double fluorescence labelling to test for co-localization of nuclei with CB1 receptor protein. Labelling was intense surrounding nuclei in the deep and superficial molecular layer; consistent with basket cell and stellate cell inhibitory interneurons that regulate depolarization-induced suppression of inhibition (DSI) of Purkinje cells.